Digital Radio Mondiale (DRM) is an open standard digital radio system for short-wave, medium-wave and long-wave communication. Audio source signals are typically encoded into digital signals and may be multiplexed with other digital data for transmission. The multiplexed audio signals and digital data may then be encoded by Quadrature Amplitude Modulation (QAM) to create Main Service Channel (MSC) cells. A Fast Access Channel (FAC) signal that contains information needed to find services and begin decoding the multiplexed signal may also be encoded by QAM to create FSC cells. Furthermore, a Service Description Channel (SDC) signal that provides information to decode services in the DRM transmission and to find alternate sources of data may also be encoded by QAM to create SDC cells.
The MSC, FAC and SDC cells may then be combined and an orthogonal frequency-division multiplexing (OFDM) signal generator used to create OFDM symbols representing the cells. The OFDM symbols may then be used to modulate a radio frequency signal for transmission to DRM receivers.
The DRM standard defines four ‘robustness modes’ of operation, intended to provide robust transmission under four types of signal propagation conditions. The transmitted DRM signal includes a succession of OFDM symbols, each symbol being made of a guard interval followed by a part of the symbol containing transmitted data. Each symbol is the sum of K sine wave portions (or carriers) equally spaced in frequency. Each sine wave portion, called a “cell”, is transmitted with a given amplitude and phase and corresponds to a carrier position. Each carrier is referenced by an index, or number.
The spacing between carrier frequencies and the number of carriers used to transmit a DRM signal are selected based upon a desired robustness mode of the signal and a desired frequency occupancy mode for the signal. The carriers are baseband signals and are used in a DRM transmitter to modulate a reference radio frequency signal.
A transmitted DRM signal is organized into transmission super frames. Each transmission super frame includes three transmission frames. A transmission frame includes a predetermined number of OFDM symbols, transmitted sequentially. The number of OFDM symbols is determined based upon a desired robustness mode, and is different for each mode. Under the DRM standard, a transmission frame may include pilot cells, control cells and data cells. The pilot cells (or reference cells) may be used for frame, frequency and time synchronization, channel estimation and robustness mode identification. Pilot cells are selected cells modulated with predetermined phases and amplitudes.
Gain reference cells (or gain pilots) are primarily used for coherent demodulation of a received DRM signal. Gain reference cells are spread throughout the OFDM symbols and may be used by a DRM receiver to estimate the channel response of the RF channel over which the DRM signal is received.